As electrical devices and systems have become increasingly prevalent in consumer and industrial applications, there has been a corresponding increase in the use of batteries. The uses of batteries to supply electrical power are as varied as the electrical devices or systems in which they are used. Some electrical systems, such as portable electronic devices, use batteries as their primary source of electrical energy. Other electrical systems or devices receive their primary supply of electrical power from a power source such as a generator, power plant, or line power supply. Even these devices often utilize batteries, however, as a back-up or secondary supply of electrical power. In such battery-backed systems, if the primary power source fails, the battery can be used to supply electrical power until the primary power supply is reinstated. This scheme of redundant power sources is often utilized in electrical devices or systems in which a temporary loss of power is problematic. Such systems include very complex as well as relatively simple applications. Examples include alarm clocks, where a loss of power could result in the clock losing track of the proper time thus resulting in a false or a late alarm; computers, where an untimely loss of power could result in lost data; and telecommunications systems, where a loss of power could result in a shutdown of communications networks.
As used herein, the term battery will include both a singular device used to store electrical energy as well as multiple storage devices connected in an array or other configuration to provide additive storage capacity. The process of storing electrical energy or power into a battery is referred to as charging the battery. Conversely, the process of removing or using the stored electrical energy from a battery is referred to as discharging the battery. In battery-backed systems, i.e., systems utilizing a non-battery power supply as a primary power supply with a battery as a back-up or secondary power supply, the primary power may be connected such that the battery will be automatically and continuously charged by the primary power supply. This ensures that the battery will be fully charged and ready for use when and if the primary power supply fails.
It should be noted that the total amount of energy which can be stored in a battery, i.e. a battery's total capacity, depends not only on the type and size of the battery, but also on the age of the battery and its operating environment during life. In application, all batteries slowly begin an ageing process which results in a continuing decrease in a battery's available capacity and in other performance characteristics. This deterioration in a battery's performance is typically caused by an increase of internal resistance in the battery caused by water loss, grid corrosion/deterioration, bad cells, or other deleterious means.
In systems which rely on batteries to supply electrical power, either as the primary or secondary power supply, battery performance is counted on and therefore must be reliable. In those electrical devices or systems in which a temporary loss of power is problematic ensuring proper battery performance can be a critical system design feature. In many systems which utilize a battery, then, it is important to monitor the condition or health of the battery. Several methods and apparatus are available for determining the condition of a battery by monitoring certain battery parameters during a battery discharge which are indicative of the battery's performance. Specifically, one approach for determining the remaining capacity (Q) and reserve time (t) of a discharging battery is disclosed in U.S. Pat. No. 4,876,513. This method takes advantage of the fact that when battery voltages (corrected for internal resistance) are plotted versus a ratio of ampere-hours remaining to ampere-hours available to a certain discharge voltage, all discharge curves fall on a single curve. The battery voltages are calculated using a battery internal resistance and discharge current that are measured periodically during discharge. An even more accurate apparatus and method of predicting remaining battery capacity and reserve time of a discharging battery to a selected end voltage is disclosed in U.S. Pat. No. 5,631,540. In this patent, the battery reserve time (t) of a discharging battery is determined by an arrangement considering the discharge current (I), battery voltage (V), battery temperature (T), and the battery's internal resistance (R.sub.int). The remaining battery capacity (Q) is determined from the ratio between a maximum theoretical capacity (Q.sub.max) and its present capacity (Q.sub.present). This normalized battery capacity value is plotted versus a temperature-corrected battery overvoltage (.eta.) to produce a discharge characteristic curve that is invariant to discharge rate, temperature, and battery size. A reserve time (t) can then be calculated from the determined capacity value (Q) using the relation: ##EQU1## Utilizing a computer, this method can provide continuing real time prediction of the remaining capacity (Q) and reserve time (t) of the battery on discharge. By comparing these performance characteristics to base or normal criteria for a new or healthy battery, one can determine the relative condition or health of the battery. Although any of these or other battery diagnostic methods can be used to measure battery performance and thus determine the condition of the battery, it should be noted that the most accurate methods of analyzing a battery's condition require monitoring the battery's performance during a discharge.
Because a battery's performance changes over time, the battery must be monitored on an ongoing basis. Since an accurate analysis of the condition of a battery requires a battery discharge, this means battery discharges must occur in frequent intervals in order to accurately monitor the condition of the battery over time. In addition to a frequency requirement, the battery discharges must be of a certain duration in order to provide enough time to collect sufficient data on the battery's performance to perform the necessary battery diagnostics. In practice, however, normal battery discharges in a system may be infrequent and/or of insufficient duration to adequately monitor the condition of the battery. In particular, systems which operate from a primary power source and use a battery for back-up power may not use the battery for days, months, or even years. In such systems, by the time a battery discharge occurs sufficient to test the battery, the battery may already be defective. Accordingly, it is often necessary to manipulate the system to initiate a battery discharge specifically in order to monitor the battery's condition.
Previously, the necessary battery discharges would be initiated by simply disconnecting or turning off the main (non-battery) power supply, such that the system would begin relying on the battery to supply electrical power. For systems in which a temporary power loss is problematic, this method presented a risky proposition, i.e. turning off the primary power supply, thereby relying on the battery to supply power, in order to test if the battery is still good. Using this method, by the time one learns that the battery is defective, the system may already have lost power causing a complete system failure.
Alternatively, the battery can be tested "off-line" meaning the battery is disconnected from the system for testing. Once the battery is disconnected from the system, a test load is connected to the battery and the condition of the battery can be monitored while discharging over the test load. Although this off-line testing method reduces the risk of a complete system failure, as opposed to simply disconnecting the main power supply and thereby causing a battery discharge, this method is typically more burdensome to perform both in terms of additional hardware required, i.e., the required test load and battery disconnect circuitry, as well as any labor required to disconnect the battery from the system. In addition, this method also presents some risk of system failure. Specifically, if the main system power supply fails while the battery is off-line and disconnected from the system, there will effectively be no battery back-up and a complete system failure can occur. Finally, it should be noted that a discharge over a test load may not accurately simulate a true battery discharge in the system, which may affect the accuracy of the results from the diagnostic testing performed.